stator resistance test -not accurate?

[verado7]

hi ,

i was diagnosing an intermittent mis that turned into the motor not being able to achieve more than part throttle - thinking stator i did the resistance test and found the low speed circuit to show 0 resistance - so i go to a reputable used merc guy and low and behold every stator in his shop tested with 0 resistance on the blue/blue-wht wires and the same 1400 ohms on the red/red-wht. so is the resistance test a non accurate way of diagnosing a stator or is ever stator in his shop dead ? anyone with a 81-88 45hp classic fifty feel free to post your findings .

 

[j_martin]

Re: stator resistance test -not accurate?

You have a faulty range in the ohm meter you're using.

 

[achris]

Re: stator resistance test -not accurate?

Measuring resistance on stators is not really a valid test. You need to measure the output, and for that you need a DVA meter, or a DVA adapter for a normal meter.

Chris.........

 

[sschefer]

Re: stator resistance test -not accurate?

You have a faulty range in the ohm meter you're using.

Yep, testing blue to ground and blue white to ground should be 6000 ohms OEM or 2250 if its a CDI stator. Red to ground or Red White to ground should be 60-150 ohms OEM or 50 ohms if its a CDI.

Not sure how your checking yours and your friends but the above is the correct resistance tests.

 

[achris]

Re: stator resistance test -not accurate?

Those particular stators are not connected to ground.

 

[sschefer]

Re: stator resistance test -not accurate?

aChris, the test is to see that the coil windings are not grounded if it's zero it's a short. My test procedures are correct.

 

[achris]

Re: stator resistance test -not accurate?

aChris, the test is to see that the coil windings are not grounded if it's zero it's a short. My test procedures are correct.

You quoted resistance readings to ground....

Yep, testing blue to ground and blue white to ground should be 6000 ohms OEM or 2250 if its a CDI stator. Red to ground or Red White to ground should be 60-150 ohms OEM or 50 ohms if its a CDI.

The correct figure to ground should be inifinity, not zero (or any other number)....

Chris.......

 

[Faztbullet]

Re: stator resistance test -not accurate?

anyone with a 81-88 45hp classic fifty feel free to post your findings .

You cant really compare as there a 2 types of flywheels, bolt in magnet and glued in and reading are different.
Bolt in magnet
Oem reading from blue to blue/white......5000-7000
Oem reading from red to red/white.........125-155
Glued in magnets
Oem reading from blue to bluw/white......3250-3650
Oem reading from red to red/white.........75-90

You really need a decent meter and a DVA adapter to test as you can check stator voltages cranking and running the engine.

 

[verado7]

Re: stator resistance test -not accurate?

lots of good replies -thanks .

however a couple posters are correct -testing this stator requires the meter to be set on ohmsx1000 and check resistance between blue/blue-wht and red/red-wht. the test was done on my meter and his -both came up with no reading on the low circuit ( blu/blu-wht wires) as did every used stator in his shop ( he is also stummped) because we even tried the same stator on a known running engine - all showed no resistance all are origonal mercury parts -and like i said i have a highspeed miss so the testing of the stator resistance didnt confirm a problem with the high speed circuit ( red/red-wht.).
so that leaves me thinking the only true test is to use a dva and check voltage with the motor running . anyone feel free to jump in here .

 

[Faztbullet]

Re: stator resistance test -not accurate?

You need a DVA meter or adapter to do most of these tests:
1) Disconnect rectifier and retest,if problem goes away test rectifier and charge windings on stator
2) Hook DVA to blu & blu/wht and run engine to problem rpm. You should see DVA voltage rise with rpm, if a shrp drop in reading before problem occurs it usually a bad stator.
3) Hook DVA to red & red/wht and you should see smooth climb in voltage and remain high thru rpm range, readings lower than the blu wires= a bad stator
4) Hook DVA to green coil outputs, you should have 150+V and rise with rpm to 300-400DVA max. A sharp drop in volts before problem occurs = bad stator. A sharp dropon less than all cylinders will be a switch box or trigger.

 

[achris]

Re: stator resistance test -not accurate?

....
2) Hook DVA to blu & blu/wht and run engine to problem rpm. You should see DVA voltage rise with rpm, if a shrp drop in reading before problem occurs it usually a bad stator....

That's the low speed stator and you WILL see a drop in output as the engine speed increases...

This is what you should see... (and it's direct form the Merc service technicians handbook)

 

[verado7]

Re: stator resistance test -not accurate?

thanks for the graph achris - you seem knowledgeable - i just recently learned that - so it is correct to say that the low speed circuit drops off at that 2000 rpm range and the high side takes over - quick question to you then is - are resistance tests on the stator then not quite accurate - i have heard from older mechanics that resistance may be off but a stator can run fine for many years just by there desighn?

 

[achris]

Re: stator resistance test -not accurate?

What you're heard is right... As a stator is just a very long length of copper wire and so the resistance of the finished product can vary with just a very small difference is wire resistance. It will even vary according to temperature. That's why checking voltage is more reliable than checking resistance.

Chris.........

 

[Faztbullet]

Re: stator resistance test -not accurate?

and it's direct form the Merc service technicians handbook)

Yep... its page 93 in my old book and next page(94) is 3/6cyl ADI diagram..

 

[sschefer]

Re: stator resistance test -not accurate?

Here's another method, This test is done with all the wires disconnected. There should be no reading (infinite) to ground. Check resistance from blue and blue/white wires. OEM reads approximately 3500 ohms. Check resitance from red to red/white wires. OEM should read about 80 ohms.

 

[Laddies]

Re: stator resistance test -not accurate?

Ohm tests are unreliable at best, anyone hanging a shingle out as a repairman without a Direct Voltage meter or DVA is as unreliable as their OHM test. If one is backyard guy or trying to repair their own engine and a OHM meter is all they have available it's a little better than guessing but not a reliable test, except when a winding is open and a test lamp will show that.

 

[j_martin]

Re: stator resistance test -not accurate?

Ohm tests are unreliable at best, anyone hanging a shingle out as a repairman without a Direct Voltage meter or DVA is as unreliable as their OHM test. If one is backyard guy or trying to repair their own engine and a OHM meter is all they have available it's a little better than guessing but not a reliable test, except when a winding is open and a test lamp will show that.

The reason for that is that a common fault is a single turn short. It's impossible to discern with an ohmmeter, but it will cut the output tremendously.

 

[Faztbullet]

Re: stator resistance test -not accurate?

That's the low speed stator and you WILL see a drop in output as the engine speed increases...

This is what you should see... (and it's direct form the Merc service technicians handbook)

That is correct and the testing is correct..I just worded it wrong. The voltage will be slowly decreasing but should not be a sudden drop. The LS winding is still creating voltage and the internal rectifier in pack is what switches from LS to HS running. At 1700-1900 rpm the voltage will still be above 150V and thats about time rectifier circuit and HS take over, the LS voltage will decrease as rpm increases but should not fall off suddenly.

If the engine starts, idles and runs then starts missing, the LS winding has to be working to some extent. Solution: Need to check DVA voltages from stator and pack.